The present invention relates, in general, to radio frequency (RF) power transistors, and more particularly, to radio frequency (RF) power transistors operating at a frequency greater than 500 megahertz and dissipating more than 5 watts of power. However, it should be understood that certain aspects of this invention have applicability at frequencies below 500 MHz and below 5 Watts. For example, it could find particular utility in power supply and power management circuitry, as well. Therefore, the term “radio frequency (RF) power semiconductor device” or “radio frequency (RF) power transistor” as used in this specification should not be construed as limiting the invention unless the claims specifically recite such limitations.
The number of wireless applications has grown significantly over the past decade. The cellular telephone market is among the most pervasive of wireless technologies. The use of wireless devices is no longer considered a luxury but has become a necessity in the modern world. Wireless is by no means limited to cellular applications. Local area networks, digital television, and other portable/non-portable electronic devices are all moving towards having wireless interconnect. Not only are the number of different types of wireless devices increasing but there is also a need for higher data content that can be transmitted and received. Increasing the content being delivered requires more bandwidth to transmit the data at a rate that is usable for the customer. For example, it is well known that most cellular telephones are currently operating with 2G (2nd generation) or 2.5G wireless infrastructure. Second generation wireless (2G) is known for the conversion from analog to digital technology for voice applications. The 2G and 2.5G wireless infrastructure has limited capability to send large amounts of data or information to a user.
Third generation cellular (3G) is an upgrade in cellular transmission capabilities to meet the demands for the transmission of higher content. An example of the higher content includes video information and real time access to the internet. One area of licensed spectrum that will be utilized for 3G is at a frequency of 2.1 GHz which will be deployed having a minimum of 144 kbps packet-data service. Furthermore, there are plans for an enhanced 3G that requires transmission in the 2.6-2.8 GHz range. Although 4G has not been defined, it is predicted that higher frequency operation will be required to provide the bandwidth needed for high data rate transmission. In particular, it is expected that 4G wireless transmission will be at frequencies greater than 3 GHz.
There are similar changes occurring in areas other than cellular, such as television transmission where the conversion to digital television is mandated by the federal government within the next decade. The simultaneous transmission of high definition television (HDTV) further increases the complexity of the RF transmission equipment. Another area that is rapidly expanding wireless activity is wireless broadband for access to the internet. What all of these applications have in common is the use of RF power transistors in power amplifiers (PA) that provide a power output from 5 watts to kilowatt levels.
The move to high frequency and high power transmission places enormous demands on the RF power transistor. RF power transistors are typically used in output stages of transmitters, for example in cellular base transceiver stations (BTS). The operating frequency for a cellular BTS can be as low as 450 MHz and as high as 2.7 GHz at this time. The power output of a cellular BTS is typically 5 watts and above. Moreover, the wireless industry is moving to standards that require better linearity and lower distortion at the higher frequency of operation. Wireless interface technologies such as WCDMA (wideband code division multiple access) and OFDM (orthogonal frequency division multiplexing) require high linearity to maximize data throughput and prevent spurious signals from being transmitted outside the transmission band.
The RF power transistor is typically used in a grounded source configuration. The predominant device being used for this type of high power radio frequency application has severe device design constraints when attempting to further extend frequency, operating voltage, and lowering distortion. Furthermore, thermal issues of the RF power transistor are as important as electrical design in a RF power amplifier and must be addressed for higher power and higher frequency operation.
Accordingly, it is desirable to provide a RF power transistor that operates at higher frequencies with increased linearity. In addition, it is desirable to provide a RF power transistor that is simple to manufacture and lower in cost. It would be of further benefit if the RF power transistor had improved thermal management, higher voltage operation and reduced parasitics.